RF power amplifiers are used in a variety of applications such as base stations for wireless communication systems etc. RF power amplifiers are designed to provide linear operation without distortion. The signals amplified by the RF power amplifiers often include signals that have a high frequency modulated carrier having frequencies in the 400 megahertz (MHz) to 4 gigahertz (GHz) range. The baseband signal that modulates the carrier is typically at a relatively lower frequency and, depending on the application, can be up to 1 GHz.
One popular power amplifier architecture currently employed is the Doherty amplifier. The Doherty amplifier was first proposed by William H. Doherty, in 1936, and is described in “A new high efficiency power amplifier for modulated waves,” Proc. IRE, vol. 24, pp. 1163-1182, September 1936, the content of which is incorporated by reference in its entirety. The Doherty amplifier employs a main amplifier which provides amplification at all power levels, and a peaking amplifier, which turns on once a high-power level threshold is crossed. Efficiency is enhanced through load modulation of the main amplifier from the peaking amplifier.
Designers of modern RF power amplifiers pay considerable attention to narrowband frequency limitations. For example, reactive components, such as capacitors, inductors, transmission lines, etc., that are employed in RF power amplifier circuit topologies including Doherty amplifiers, to provide phase shifting (e.g., for power combination) and impedance matching, represent one source of frequency limitation. These reactive components introduce a frequency dependence in the circuit whereby optimum efficiency is achieved only at a particular frequency value, and high efficiency is achieved only in a narrowband range. Accordingly, designers are seeking ways to improve the wideband impedance response of RF power amplifier circuit topologies without sacrificing power efficiency.